The present invention relates to a propeller shaft for transmitting rotational power from the transmission to the differential in a four-wheel drive vehicle (4WD vehicle), front engine rear drive vehicle (FR vehicle), or the like. Although a propeller shaft of two-joint type is typical, a propeller shaft of three-joint type, four-joint type, or the like is also used according to the structure of the vehicle and the required characteristics.
At present, except for some luxury vehicles, it is the mainstream to use a Cardan joint (joint using a spider) for the propeller shaft of a four-wheel drive vehicle (4WD vehicle) or front engine rear drive vehicle (FR vehicle). Due to the non-constant velocity of the Cardan joint, however, the noise, vibration and harshness commonly known as NVH characteristics of the vehicle deteriorate. As means for improving the NVH characteristics, there is a tendency to use a constant velocity universal joint for the propeller shaft.
FIG. 6 shows a Rzeppa constant velocity universal joint (ball fixed constant velocity universal joint) used for a conventional propeller shaft. The constant velocity universal joint comprises: an outer joint member 11 in which six curved guide grooves 11b are formed in the axial direction on an spherical inner surface 11a; an inner joint member 12 in which six curved guide grooves 12b are formed in the axial direction on an spherical outer surface 12a and an engagement portion 12c having teeth (serration or spline) is formed on an inner surface; six torque transmitting balls 13 disposed in six ball tracks formed by the cooperation between the guide grooves 11b of the outer joint member 11 and the corresponding guide grooves 12b of the inner joint member 12, respectively; and a cage 14 for holding the torque transmitting balls 13.
The center O1' of the guide groove 11b of the outer joint member 11 is offset from the spherical center of the inner surface 11a, and the center O2' of the guide groove 12b of the inner joint member 12 is offset from the spherical center of the outer surface 12a, by an equal distance in the opposite axial directions (the center O1' is offset to the left side in the diagram and the center O2' is offset to the right side in the diagram). The ball tracks formed by the cooperation between the guide grooves 11b and the corresponding guide grooves 12b have therefore the shapes such that the ball tracks are widened toward one side in the axial direction (left side in the diagram) like wedge shapes. Both of the spherical center of the inner surface 11a of the outer joint member 11 and the spherical center of the outer surface 12a of the inner joint member 12 lie within the joint center plane O' including the center O3' of the torque transmitting ball 13.
When the outer joint member 11 and the inner joint member 12 is displaced with each other by an angle .theta., the torque transmitting balls 13 guided by the cage 14 are maintained in the two-equally divided plane (.theta./2) of the angle .theta. at any operating angle .theta., so that the constant velocity of the joint is assured.
A constant velocity universal joint has been often used for a drive shaft in the power transmitting mechanism of a vehicle. The conventional constant velocity universal joint for a propeller shaft is adopted the very design of the conventional constant velocity universal joint for a drive shaft. However, when the characteristics of the power transmission of the joint for the propeller shaft are compared with those of the joint for the drive shaft, the torque loaded on the propeller shaft is about the half of that loaded on the drive shaft and the practical range of the operating angle of the propeller shaft is narrower than that of the drive shaft. Consequently, the conventional specification adopting the design for the drive shaft as it is more than the required characteristics. It has room for improvement from the viewpoint of further reduction of the weight, size, and cost. Since the propeller shaft rotates higher than the drive shaft, it is preferable that the joint part is more compact also from the viewpoint of increase in the rotational speed.